The group III-nitride family of materials can be fabricated into optoelectronic devices that emit or absorb light in the ultraviolet (UV) and the entire visual spectrum. There is a large commercial market for, for example, GaN-based light emitting diodes (LEDs) and edge-emitting lasers that emit light in the UV, blue and green wavelength ranges. In the future it is expected that a number of additional devices, including vertical cavity surface emitting lasers (VCSELs) and resonant cavity light emitting diodes (RCLEDs) will be commercialized based on the group III-nitride material.
Deposition of group III-nitride films is typically conducted on expensive silicon carbide (SiC) or sapphire, i.e., aluminum oxide, substrates. SiC and sapphire substrates represent a significant portion of the overall cost to fabricate a LED. Group III-nitride-based electronic devices are typically fabricated on SiC substrates due to the high thermal conductivity of SiC. Unfortunately, SiC is not available in large diameters and is exceedingly expensive particularly for high-purity, insulating SiC, which is the preferred substrate for a group III-nitride high electron mobility transistors (HEMTs).
Epitaxy of high-quality group III-nitride films on silicon (Si), rather than SiC, is difficult due to the large lattice and thermal expansion coefficient mismatch between the group III-nitride film and the Si substrate. For example, volume production of GaN LEDs deposited on Si substrates is generally plagued with high defect and crack density in group III-nitride films because of the large lattice and thermal expansion mismatch with the Si substrate.
Conventionally, GaN-based structures have been grown on Si by first depositing a buffer layer on the Si substrate such as a thick AlN layer of about 25 to 300 nm, ZnO, TiN, SiC, and others. Using such a buffer layer, a superlattice (SL), which for example may be a distributed Bragg reflector (DBR), which is designed to reduce the defect density of a GaN layer has been placed away from the substrate, for example either on a buffer layer or on a GaN/buffer layer. However, the addition of such a SL or buffer layer, while allowing for a crack free and reduced defect GaN layer, causes distorted reflectance because of the buffer layers presence and thus is not particularly well suited for optoelectric applications such as an LED.